Doc updates.

docs/index.md

@@ -464,6 +464,72 @@ class Settings(BaseSettings):
     So if you provide extra values in a dotenv file, whether they start with `env_prefix` or not,
     a `ValidationError` will be raised.
 
+## Parsing default objects
+
+Pydantic settings uses copy-by-reference when merging default (`BaseModel`) objects from different sources. This ensures
+the original object reference is maintained in the final object instantiation. However, due to an internal limitation,
+it is not possible to partially update a nested sub model field in a default object when using copy-by-reference; the
+entirety of the sub model must be provided.
+
+This behavior can be overriden by setting the `default_objects_copy_by_value` flag to `True`, which will allow partial
+updates to sub model fields. Note of course the original default object reference will not be retained.
+
+```py
+import os
+
+from pydantic import BaseModel, ValidationError
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class SubModel(BaseModel):
+    val: int = 0
+    flag: bool = False
+
+
+ORIGINAL_OBJECT = SubModel()
+
+
+class SettingsCopyByReference(BaseSettings):
+    model_config = SettingsConfigDict(env_nested_delimiter='__')
+
+    default_object: SubModel = ORIGINAL_OBJECT
+
+
+class SettingsCopyByValue(BaseSettings):
+    model_config = SettingsConfigDict(
+        env_nested_delimiter='__', default_objects_copy_by_value=True
+    )
+
+    default_object: SubModel = ORIGINAL_OBJECT
+
+
+by_ref = SettingsCopyByReference()
+assert by_ref.default_object is ORIGINAL_OBJECT
+
+# Apply a partial update to the default object using environment variables
+os.environ['DEFAULT_OBJECT__FLAG'] = 'True'
+
+try:
+    # Copy by reference will fail
+    SettingsCopyByReference()
+except ValidationError as err:
+    print(err)
+    """
+    1 validation error for SettingsCopyByReference
+    nested.val
+      Field required [type=missing, input_value={'flag': 'TRUE'}, input_type=dict]
+        For further information visit https://errors.pydantic.dev/2/v/missing
+    """
+
+# Copy by value will pass
+by_val = SettingsCopyByValue()
+assert by_val.default_object is not ORIGINAL_OBJECT
+
+print(by_val.model_dump())
+#> {'default_object': {'val': 0, 'flag': True}}
+```
+
 ## Command Line Support
 
 Pydantic settings provides integrated CLI support, making it easy to quickly define CLI applications using Pydantic

pydantic_settings/sources.py

@@ -262,12 +262,14 @@ def __init__(self, settings_cls: type[BaseSettings], default_objects_copy_by_val
             if default_objects_copy_by_value is not None
             else self.config.get('default_objects_copy_by_value', False)
         )
-        if self.default_objects_copy_by_value:
-            for field_name, field_info in settings_cls.model_fields.items():
-                if is_dataclass(type(field_info.default)):
-                    self.defaults[_field_name_for_signature(field_name, field_info)] = asdict(field_info.default)
-                elif is_model_class(type(field_info.default)):
-                    self.defaults[_field_name_for_signature(field_name, field_info)] = field_info.default.model_dump()
+        for field_name, field_info in settings_cls.model_fields.items():
+            if not self.default_objects_copy_by_value:
+                if is_dataclass(type(field_info.default)) or is_model_class(type(field_info.default)):
+                    self.defaults[_field_name_for_signature(field_name, field_info)] = field_info.default
+            elif is_dataclass(type(field_info.default)):
+                self.defaults[_field_name_for_signature(field_name, field_info)] = asdict(field_info.default)
+            elif is_model_class(type(field_info.default)):
+                self.defaults[_field_name_for_signature(field_name, field_info)] = field_info.default.model_dump()
 
     def get_field_value(self, field: FieldInfo, field_name: str) -> tuple[Any, str, bool]:
         # Nothing to do here. Only implement the return statement to make mypy happy